Converter circuits are used in a multiplicity of applications. For example, a converter circuit whose voltage can be scaled particularly easily is specified in the WO 2007/023064 A1, the disclosure of which is hereby incorporated by reference in its entirety. In this document, the disclosed converter circuit has a first and a second partial converter system, wherein the partial converter systems are connected in series to one another via two series-connected inductances. A junction point between the two series-connected inductances forms an output connection for example for an electrical load. Each partial converter system has at least one two-pole switching cell, wherein, when there are a plurality of switching cells in one partial converter system, these switching cells are connected in series to one another. Each two-pole switching cell has two series-connected controllable bidirectional power semiconductor switches with a controlled unidirectional current flow direction, and a capacitive energy store which is connected in parallel with the series circuit formed by the power semiconductor switches.
FIG. 1 shows a known apparatus for operation of a converter circuit according to WO 2007/023064 A1, which has a first control circuit for production of a control signal for controlling the power semiconductor switches in the switching cells of the first partial converter system, and has a second control circuit for production of a further control signal for controlling the power semiconductor switches in the switching cells of the second partial converter system.
The converter circuit according to WO 2007/023064 A1 can be operated such that a pure AC voltage and a pure alternating current are produced at the output connection. The capacitive energy stores for the switching cells are designed such that the voltage ripple on the capacitive energy stores remains within a predetermined fluctuation range for a given maximum current at the output connection and for this current being at a given frequency. If a lower frequency is desired than that on which the design was based, then the voltage ripple rises. If the intention is to produce a direct current or an alternating current with a direct-current component at the output connection, then the voltage ripple rises to virtually infinity. The capacitive energy stores in this case could either be fed externally or be selected to be infinitely large in order to avoid being completely discharged or indefinitely overcharged during operation with direct current or a direct-current component at the output connection.
The operation of a converter circuit according to WO 2007/023064 A1 does not allow the capacitive energy stores to be designed independently of the desired current at the output connection (i.e., independently of the frequency of the current).